The invention relates to an X-ray tube with a rotary anode usable in the general field of radiology and more particularly appropriate for the case where the rotary anode is subject to great accelerations. It also relates to a process making it possible to fix a rotary anode to a support shaft.
The rotary anodes of X-ray tubes are generally shaped like a disk. The disk is fixed to a shaft, which is itself connected to the rotor, the assembly being rotated by a rotary magnetic field to which the rotor is exposed. The rotating rotary anode is exposed to very high thermal and mechanical stresses.
Thus, the X-radiation is obtained under the action of electron bombardment of a small surface of the anode and a very small part of the electrical energy used for accelerating the electrons is converted into X-rays. The rest of this energy is dissipated as heat in the rotary anode. In addition, the rotary anode is exposed to very significant thermal shocks and can reach very high temperatures. The mechanical stresses are particularly lined with high rotation speeds and high accelerations to which the rotary anode is exposed.
Generally the anode is fixed to the shaft connecting it to the rotor by fixing means acting by gripping. Under the effect of the aforementioned stresses, the rotary anode tends to be loosened and move during rotation with respect to its support shaft. This can lead to an unbalance of the rotary anode - rotor assembly, with the appearance of vibrations and risks of mechanical breaks.
This problem of fixing the rotary anode to the shaft connecting it to the rotor exists with all types of rotary anodes. However, this problem is even more critical in the case of graphite anodes, due to differences between the expansion coefficients of graphites and the support shaft, and on the friction coefficient of graphite, which is a material having a lubricating tendency.
Examples are given below of various methods attempting to obviate this problem.
(a) pins or keys engaged in the anode and support shaft, according to transverse axes with respect to the latter, but this solution is not very effective in the case of graphite anodes, due to the friable character of graphite;
(b) support shafts provided with off-centered bosses, but this solution suffers from the disadvantage that it leaves a very large mechanical clearance between the anode disk and the support shaft;
(c) a further very different solution consists of brazing the anode on its support shaft. This solution ensures a good connection between anode and support shaft, although the operation is difficult to perform. In addition, it can limit the performances of the X-ray tube by reducing the quality of the vacuum existing in the latter, if the operating temperature leads the brazing materials to have an excessive vapor tension. It is also pointed out that this fixing by brazing prevents any subsequent disassembly;
(d) European Patent Application No. 0 055 828 describes the construction in the same graphite block of the anode disk and its support shaft, in order to transfer the graphite - metal junction into a lower temperature zone, as it is further away from the anode disk. Apart from its very high cost, this configuration is mechanically very fragile, due to the limited mechanical strength of graphite.
(e) French Patent Application No. 2 467 483 describes a construction in which a pyrolitic graphite sleeve is brazed into the graphite anode disk body. However, this solution is very expensive to perform, due to the difficult and mechanically fragile construction.
This list of the various methods aiming at fixing the rotary anode to its support shaft shows that the problem caused by this fixing is of great importance to all X-ray tube designers. It also shows that this problem has not been satisfactorily solved.